For a drastic energy conservation in data centers (more than 30%), a new advanced concept of thermal management systems of integrated cooling network in server racks is proposed, and a series of developmental studies toward the realization of the concept have been conducted. The concept consists of the following technological items: - Plug-in thermal network cables, - Narrow channel heat exchangers (single-phase and two-phase), - Thin flat-type heat pipes, - Development of nano-fluids for heat transfer enhancement. In this concept, CPUs in server racks are directly cooled with the aid of advanced 1.5 mm thick flat-type heat pipes or 3 mm thick narrow channel heat exchangers, though CPUs in server racks, at present, are cooled down with massive heat sinks by forced air flow in the racks. In the case of heat pipes, condensation regions are cooled by either single-phase or two-phase narrow channel heat sink instead of air-cooled finned heat sink. These cooling units are accommodated in server racks. In addition, plug-in thermal network cables, in which a working fluid for narrow channel heat exchangers is running, are integrated among server racks, and thermal energy dissipated in server racks are inclusively merged and managed by the thermal network cables. The working fluid heated up by the dissipated heat is recuperated in an outdoor cooling system. An attempt at the applications of silver nano-fluids as a working fluid in heat pipes and thermal network is also conducted in the present study. Silver nano-fluids are thermochemically synthesized by a microwave heating technique, which allows for superior stable suspension characteristics.